This electrodialysis apparatus can be used in a number of large scale process applications. Specifically, it may be used for the recovery of lactic acid from fermentation derived ammonium lactate in a two compartment cation cell. There may be either a nanofilter or an ion exchange column (or both) in communication with the base loop of the cation cell. The column contains a weak acid cation exchange resin.
For more information on the background of the inventive structure, reference may be made to my co-pending applications having the following identifications: Process for the Recovery of Organic Acids and Ammonia from Their Salts, Ser. No. 08/639,831, filed Apr. 29, 1996; now U.S. Pat. No. 5,814,489 Electrodialysis Apparatus, Ser. No. 08/784,050 filed Jan. 17, 1997 now U.S. Pat. No. 5,972,191 and Gasket and Apparatus for Deionization, Ser. No. 08/785,648, filed Jan. 17, 1997 now U.S. Pat. No. 6,123,823
The invention includes an apparatus and its related method using an electrodialysis cell (or cells) in combination with a nanofiltration unit for filtering an incoming monovalent salt solution in order to minimize the level of multivalent impurities. The apparatus may also include an electrodialysis cell (or cells) in combination with an ion exchange column in communication with a base loop of the said electrodialysis cell. The pH of the base product from the apparatus is preferably in the range of 7 to about 13.5.
The apparatus and process are particularly well suited to the production of acids, especially organic acids, in conjunction with weak bases such as ammonia, or the salts of weak acids such as sodium carbonate or sulfite. The electrodialysis cells of the invention may employ membranes, such as bipolar membranes, or assemblies for splitting water. Alternatively the splitting of water for acid, base production may be accomplished with a set of electrodes.
1.Field of the Invention
This invention relates to apparatus and processes for electrodialysis of salts and more particularly to apparatus and processes that incorporate at least one of two distinct features: a nanofiltration unit combined with an electrodialysis unit and an ion exchange column connected to and in communication with the base loop of an electrodialysis cell.
2. Description of Related Art
Fermentation processes for producing organic acids, such as acetic and lactic acids, go through an intermediate production of salts, such as ammonium acetate or lactate. Hence, salts are the byproducts or intermediate products of a number of chemical processes. For example, regenerable flue gas desulfurization processes use a sodium alkali to absorb the SO2, thus resulting in a soluble bisulfite salt, NaHSO3. Production of soda ash (Na2CO3) requires the processing of the raw material salt viz., trona (Na2CO3.NaHCO3.2H2O) or the naturally occurring brines. In magnetohydrodynamic power generation processes the potassium carbonate seed material absorbs SO2 in the fuel and is converted to a byproduct potassium sulfate.
Electrodialysis (ED) may be used to convert these and other soluble salts directly into their acid and base components. For example, such a procedure enables a direct recovery of a relatively pure form of the organic acid from its organic salt. The co-product base (ammonia for example) may be recovered for reuse in the fermentation process for pH adjustment, thus permitting an economical and environmentally superior option for producing organic acids. In other instances, such as with sodium bisulfite, trona or potassium sulfate, the electrodialysis offers an environmentally superior route for recovering or recycling the acid, base components.
Electrodialysis uses direct current as a means for causing a movement of ions in the solutions of the processing streams of salt starting material. Electrodialysis processes are usually carried out in an arrangement comprising a stack where a plurality of flat sheet ion exchange membranes and gasket sheets are clamped together. These sheets provide flow paths for containing salt materials that produce acids and bases. The process unit requires a means for splitting water into hydrogen (H+) and hydroxyl (OHxe2x88x92) ions.
Two useful means for splitting water are:
(1) A bipolar membrane or a bipolar module formed by a combination of cation and anion membranes which functions as a bipolar membrane. Suitable bipolar membranes are available from Aqualytics, a division of Graver Water, and from Tokuyama Soda, and from the Formic Corporation; and
(2) An electrode set comprising an anode and a cathode. The electrodes, particularly the anodes, are coated for chemical stability, for minimizing power consumption, and for the formation of byproducts other than hydrogen (at the cathode) and oxygen (at the anode), among other things. Suitable electrodes are available from the Eltech Corporation, the Electrode Products Inc., and from others. A hydrogen depolarized anode can also generate H+ ions in an aqueous solution of an electrode stream next to the anode.
As described in my above-identified co-pending applications, the stack contains electrodes (anode and cathode) at either end and a series of membranes and gaskets which have open active areas in their middle to form a multiplicity of compartments which are separated by the membranes. Usually, a separate solution (an electrode stream) is also supplied to each of the compartments containing the electrodes. Special membranes may be placed next to the electrodes to prevent a mixing of the process streams with the electrode streams.
The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. Each of the repeating units is called the xe2x80x9cunit cellxe2x80x9d or simply a xe2x80x9ccell.xe2x80x9d The solution is supplied to the compartments by internal manifolds formed as part of the gaskets and membranes or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell.
Streams of processing fluids may be fed from one stack to another in order to optimize process efficiency. After one pass through the stack, if the change in the composition of a process stream is relatively small, the process solutions can be recycled by being pumped to and from recycle tanks. An addition of fresh process solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to maintain the concentration of products within a desired range.
When bipolar membranes are used to form acid or base from the salt, in order for the membrane to function as a water splitter, the component ion exchange layers must be arranged so that the anion selective layer of each membrane is closer to the anode than the cation selective layer. A direct current passed through the membranes in this configuration causes water splitting with OHxe2x88x92 ions being produced on the anode side and a corresponding number of H+ ions being produced on the cathode side of the membranes. The dissociated salt anions move toward the anode. The dissociated salt cations move toward the cathode.
The electrolysis process works in a similar manner, with the water splitting occurring at the two electrodes. When a direct current appears, water molecules are converted to oxygen gas at the anode along with the introduction of H+ ions into the aqueous solution. At the cathode, the water molecules are converted to hydrogen gas along with the introduction of OHxe2x88x92 ions into the aqueous solution. In the hydrogen depolarized anode based electrolysis unit, OHxe2x88x92 ions are released into the aqueous solution next to the cathode. While released, the hydrogen gas is forwarded to the catalytic hydrogen depolarized anode for H+ ion generation.
Electrodialysis equipment for acid/base production may have three compartment cells comprising bipolar, cation and anion membranes; two compartment cells containing bipolar and cation (or anion) membranes; multichamber two compartment electrodialysis cells comprising bipolar and two or more cation membranes. The term xe2x80x9cbipolar membranexe2x80x9d also includes bipolar equivalent structures, such as the use of electrodes and composite bipolars. FIG. 1 shows the unit cell for the three most useful configurations.
Specific references are:
xe2x80x9cElectrodialysis Water Splitting Technologyxe2x80x9d by K. N. Mani; J. Membrane Sci., (1991), 58, 117-138
U.S. Pat. Nos. 4,082,835; 4,107,015; 4,592,817; 4,636,289; 4,584,077; 4,390,402; and 4,536,269.
In accordance with an aspect of this invention, an electrodialysis apparatus is improved through the addition of a nanofiltration unit upstream of an electrodialysis cell or through the use of a downstream ion exchange column in combination with the electrodialysis cell and in communication with the base loop of the cell. Or both a nanofiltration and an ion exchange column may be used.